1. Field of the Invention
This invention is related to the field of voltage-controlled oscillators.
2. Description of the Related Art
A voltage-controlled oscillator (VCO) comprises circuitry that receives a control voltage and outputs an oscillating signal having a frequency that is controlled by the magnitude of the control voltage. VCOs have a variety of uses in electronics. For example, VCOs are frequently a component of phase-locked loops (PLLs), which are often used to generate clocks in integrated circuits. The output of the VCO is the clock signal used in the integrated circuit, and the control voltage may be generated by other circuitry in the PLL to phase lock the clock signal to an input reference clock.
As the process features (e.g. channel length, oxide thickness, etc.) of integrated circuit manufacturing processes have continued to shrink over time, the magnitude of the supply voltage provided to the integrated circuit has also been shrinking. Presently, supply voltage magnitudes of around 1 volt are being used, and supply voltage magnitudes less than 1 volt are possible. Additionally, the operating frequency of the integrated circuits has continued to increase. Still further, wide ranges of operating frequencies are being supported in many integrated circuits (e.g. to permit lower frequency operation during periods of lower activity, for power management reasons). Accordingly, the range of frequencies that the VCO in a PLL must support is large, and the range of the control voltage (bounded by ground and the supply voltage) is small. A high gain (in terms of frequency/volt) VCO is thus required.
Unfortunately, a high gain VCO provides large frequency changes in response to small changes in the control voltage, which contributes to the “jitter” experienced on a PLL-generated clock when the clock is phase-locked. Even when the clock is phase-locked, small changes in the control voltage may be expected as the circuit reacts to various transient conditions, noise, etc. These small changes, in a high gain VCO, may produce undesirable amounts of jitter.
In some PLLs, multiple VCOs are included. Each VCO has a different subrange within the desired frequency range, and thus can have a lower gain within its frequency range. Using multiple VCOs increases the size of the PLL. Typically, the subranges must be overlapped to ensure that any frequency can be adequately supported, further increasing the size for a desired frequency range for the PLL. Additionally, switching between VCOs must be supported as the operating frequency of the integrated circuit changes. Accordingly, the frequency range of such a VCO is not truly continuous.
In other PLLs, a single VCO may be used with a programmable frequency range. For example, the VCO may have a register or input control signals which identify the desired range. Again, the frequency range of the PLL is not truly continuous since the programming of the VCO must be explicitly changed (e.g. by software) as the desired frequency for the PLL is changed.